1) Field of the Invention
The present invention relates to a multiplex radio transmitter and multiplex radio transmission method, a multiplex radio receiver and multiplex radio receiving method, and a multiplex radio transceiver and multiplex transmission/receiving system suitable for use with a trunk multiplex radio communications system.
2) Description of the Related Art
In recent years, when large volumes of information including such as video data are to be transmitted at high speed, a synchronous multiplex transfer mode (Synchronous Transfer Mode) based on a SDH (Synchronous Digital Hierarchy) has been widely used as a method of transmitting information data. Multiplex radio transceivers are usually located on high ground and relay multiplexed trunk signal data at microwave band through use of a synchronous multiplex transfer mode
With regard to frequency bands, the allocation of available channels in individual frequency bands is specified in detail in the form of ITU-R (International Telecommunications Union Radio Communications Sector) recommendations.
The trunk multiplex radio communications system occupies a frequency band called C-band (3.4 GHz to 8.5 GHz), and this C-band is further classified into sub-bands: that is, a 4G band, a 5G band, an L6G band, a U6G band, a 7G band, and an L8G band. As shown in FIGS. 10 through 15, the allocation of individual channels within each of the sub-bands is also specified. The 3.5 G band has been specified only in that the ends thereof have been determined as 3.4 GHz and 3.6 GHz, and details of allocation of channels within the frequency band are still under discussion and have not yet been publicly announced.
Although a modulation scheme used for a trunk multiplex radio communications system within a microwave band has not yet been specified by ITU-R recommendations, enterprises adopt a 64 or 128 quadrature amplitude modulation scheme. This is because a large volume of data must be transmitted within the foregoing frequency bands while fulfilling the specifications, such as channel allocations, a per-channel occupied bandwidth, and adjacent channel power, and therefore there is adopted a QAM scheme having a superior frequency efficiency (a transmission rate per 1 Hz).
FIG. 23 shows the principal elements of a transmission section disposed in a transmitter of an existing trunk multiplex radio communications system. A multiplex radio transmitter 38 shown in FIG. 23 transmits a plurality of channel signals having different frequencies while converting them into a multiplexed signal and comprises a plurality of transmission sections 31 corresponding to the individual channels, an antenna 36, and an antenna diplexer 37.
In each transmission section 31, signal data (e.g., STM-1 transmitted over an SDH network) received from a synchronous multiplex repeater or a transmission-end apparatus are subjected to a baseband processing treatment in a baseband processing section (not shown). The thus-processed data are modulated and transmitted to a radio circuit. The transmission section 31 comprises a modulation section 32, a digital-to-analog converter 33, a frequency converter 34, and a band-pass filter 35.
The modulation section 32 modulates multiplexed trunk signal data and comprises a mixer 32a-1, a mixer 32a-2, a 90.degree. phase shifter 32b, a local oscillator 32c, and a hybrid section 32d.
More specifically, I-channel signal data are multiplied by an output signal of 70 MHz outputted from the local oscillator 32c. The mixer 32a-2 multiplies Q-channel signal data by an outputted from the mixer 32a-2 which is produced by phase-shifting the outputted from the local oscillator 32c through 90.degree. by means of the 90.degree. phase shifter 32b. As a result, the signal data items are modulated into a frequency band centered at 70 MHz and are combined into a single waveform by the hybrid section 32d. The value of 70 MHz represents an interval between the local frequency and the radio frequency (RF).
The digital-to-analog converter 33 converts a digital signal outputted from the modulation section 32 into an analog signal.
Further, the frequency converter 34 up-converts the thus-modulated signal having a 70 MHz band into C-band within the microwave band. The mixer 34a multiplies an output from the digital-to-analog converter 33 by a microwave carrier which ranges in frequency from about 3 GHz to 8 GHz and is outputted from the local oscillator 34b.
For each channel, the band-pass filter 35 limits a transmission band of a modulated signal in a microwave band outputted from the frequency converter 34 in such a way as to fulfill specifications defined for an air interface.
The antenna diplexer 37 combines together the band-limited signals outputted from the band-pass filter 35.
The antenna 36 transmits a radio signal outputted from the antenna diplexer 37.
Turning to a method for communications between a pair of multiplex radio repeaters, there has been employed an FDM (Frequency Division Multiplexing) method of enabling simultaneous use of a plurality of channels by a transmit-receive channel of each radio receiver being allocated a different frequency. FIG. 24 shows an example in which each pair of multiplex radio repeaters is allocated a different channel within C-band. The frequency band shown in FIG. 24 corresponds to a U6G band within the C-band and represents a group of radio frequency (RF) channels and a group of local frequencies.
Channels 1 (6,460 MHz) to channel 8 (6,740 MHz) are allocated to a downlink, whereas channel 1' (6,800 MHz) to channel 8' (7,080 MHz) are allocated to a corresponding uplink, thus enabling selection of eight possible channels.
There is overlap between the group of radio frequency (RF) channels and the group of local frequency channels. To prevent spurious radiation, the transmitter must eliminate the local frequency signals so as to prevent these signals from being emitted over the group of radio frequency (RF) channels, through use of a filter having a narrow bandwidth.
In contrast, a receiver has a receiving device having the same frequency as that transmitted from an opposing repeater. As show in FIG. 25, the receiver must separate a channel being used from the eight possible channels through use of the band-pass filter having a narrow bandwidth.
Here, the term "opposing" signifies a repeater with which the current repeater is in communication through use of a downlink channel and an uplink channel corresponding thereto. The term "opposing" will be used herein in the same sense.
To this end, the band-pass filter 35 provided in the transmission section 31 is required to have a narrow band as its band-pass characteristics. Accordingly, to select the channel being used the band-pass filter of the opposing receiver is also required to have a narrow band as its band-pass characteristics.
However, according to the existing technique, a narrow-band filter having its center frequency in a microwave band such as that mentioned previously is very expensive and bulky.
Further, since the individual transmission and receiving sections disposed in the multiplex radio transceiver transmit data through use of a plurality of channels of different frequencies, there is a need for a band-pass filter having band-pass characteristics corresponding to each of the channels, thus impeding realization of a band-pass filter capable of being used in all the channels.
Moreover, data are transmitted between the repeaters through use of a plurality of channels having different frequencies. Even in such a case, there is a need for a band-pass filter having band-pass characteristics corresponding to each of the channels, thus impeding realization of a band-pass filter capable of being used in all the channels.